Analyzing the Sheep Genome for Parasite Resistance

Agricultural Research Service (ARS) scientists and their collaborators at the International Livestock Research Institute (ILRI) in Kenya have discovered the genetic source for resistance to a parasitic nematode that infects sheep.

The researchers are the first to detect quantitative trait loci (QTL), genetic locations on chromosomes, for resistance to gastrointestinal nematode parasites in a double-backcross population derived from African native sheep. The parasites, common in tropical regions, cause significant economic and production losses in Africa each year. Sheep infected with parasites suffer from diarrhea, anemia, weight loss and sometimes death.

Geneticist Tad Sonstegard at the ARS Henry A. Wallace Beltsville Agricultural Research Center in Beltsville, Md., and researchers at ILRI hope to identify genes that increase tolerance to parasites and improve production of grazing animals. ARS is the chief intramural scientific research agency of the U.S. Department of Agriculture (USDA), and this research supports USDA’s priority of promoting international food security.

In one study, researchers mapped the regions of the genome that control resistance to gastrointestinal nematode parasites in a sheep population bred by ILRI. Hybrid rams were produced by mating a Red Maasai, which is tolerant to gastrointestinal parasites, to a Dorper, a breed that is more susceptible to the parasite. Several of the hybrid ram offspring were then bred to either Red Maasai or Dorper ewes to complete the backcross.

Scientists genotyped 20 percent of the backcross progeny to map QTL that affect parasite-resistance traits. Blood packed-cell volume and fecal egg count—indicators of parasites—were collected for three months from more than 1,060 lambs that grazed on parasite-infected pastures. Scientists selected lambs for genotyping based on parasite indicators. They detected significant QTL for average fecal egg count and packed-cell volume on chromosomes 3, 6, 14, and 22.

Future studies will focus on genotyping the same animals using the OvineSNP50, according to Sonstegard. The OvineSNP50 is a powerful tool that can examine more than 50,000 locations in the genome.

Findings from this research were published online in Animal Genetics in May 2011.

Read more about this research in the October 2011 issue of Agricultural Research magazine.